1. Field of Invention
This invention relates generally to processing of polyolefins, and more particularly processing through a die of polyethylene polymers and copolymers.
2. Description of the Relevant Art
The processing of polyethylene and extrusion of polyethylene or polyolefins is a multi-talented skill long studied in the art. There are many different parameters that influence whether a polymer is processable or not at high speeds. Such parameters include the melt index (MI), the molecular weight (Mw), the molecular weight distribution (Mw/Mn) and such. For example, it is assumed in the art that low MI polymers need high temperatures for processability, and if these conditions are not maintained, then the extrudate will have defects. Likewise even at low shear rates, high molecular weight, low MI polyethylenes extruded through a die produce a rough surface extrudate considered defective, typically called sharkskin or melt fracture. Appearance of this surface defect usually sets the maximum throughput for most processing equipments, such as film-blowing, profile extrusion or film-casting. Thus, the processing may go no faster than that point at which the sharkskin melt fracture typically appears. Some polyethylenes, for example HDPE, are also known to show what is called a "slipstick transition" when extruded at very high shear rates, which are too high for most practical purposes, typically over 1000, even above 2000 sec.sup.-1. This slipstick transition is the point at which the sharkskin or stick region converts to a slip or fast-moving wavy extrudate surface and is typically characterized by a pressure fluctuation over time. Some polymers exhibit sharp slipstick transitions characterized by high pressure fluctuations when pressure is compared to time. Other polymers do not display harsh slipstick transitions. For example, LDPE is not believed to undergo a typical slipstick transition, and if it does, it is greatly above a shear rate of 2000 sec.sup.-1.
When increasing the shear rate of a polymer extruded through a die, several transitions may be observed. For example, at low shear rate, the extrudate is smooth, and the shear stress is typically constant with time at any particular shear rate. This is the traditional operating range where most polymers are processed in industry today. With increasing shear rate, the extrudate surface becomes matt then deformed. This is typically called the sharkskin region and like the "smooth" region, shear stress is apparently constant with time at any particular shear rate. In the literature polyethylene is reported to display sharkskin beginning at shear rates for which the shear stress is higher than about .about.0.2 MPa.
At high shear stress, typically 0.5 MPa and above, the extrudate surface is wavy and highly distorted. This region is often called the wavy region or gross distortion region.
For some polymers, one or two additional regions are observed. One is called the slipstick region, and is typically characterized by the following: (1) the shear stress versus shear rate is essentially flat; (2) the measured pressure oscillates periodically over time; and/or (3) the surface of the extrudate consists of two distinct regions, one (typically sharkskin) corresponding with increasing pressure, and one (typically smooth) corresponding with decreasing pressure. The actual surface appearance depends upon the shear stress at which slipstick occurs. At shear rates above the slipstick region, and as long as the shear stress is below the onset of the wavy region, the extrudate is smooth and glossy. This is called the glossy region.
The glossy region is an acceptable operating region in terms of extrudate surface appearance, and extrusion stability. It is highly desirable in terms of throughput and melt viscosity (high throughput, low melt viscosity). Unfortunately, the shear rates required to be in the glossy region are usually too high for most practical application such as film/coating applications. Likewise, the shear stress at which the slipstick occurs is also typically too high, so that the glossy region is not observed, and the extrudate is wavy at shear rates immediately above the slipstick region.
U.S. Pat. No. 4,859,398 to Su, herein describes a method to process polyethylene, particularly LLDPE, at high shear rates, preferably above 1000 sec.sup.-1. By reducing the extrusion temperature to just above the melting temperature of the polyethylene (about 140.degree. C.) and processing at shear rates above 1000 sec.sup.-1, Su found a smooth surface in the extrudate.
U.S Pat. No. 2,991,508 to Fields, discloses the "super extrusion" of high density polyethylene at melt temperatures over 235.degree. C. and shear rate up to 8,000 sec.sup.-1. The super extrusion is disclosed to vary with die temperature.
Speeds above 1000 sec.sup.-1 are impractical, however, for most polyolefin processing. Thus it would be desirable to develop a process for processing polyolefins at speeds faster than those currently in commercial use but less than 1000 sec.sup.-1 without obtaining sharkskin melt fracture.